It is interesting that, not many people know how polio is spread in a community. Before trying to understand it, let us first learn what is polio infection. Also we will root out, what this infection is capable of doing to a human body. 

What is polio ? What are types of polio virus ?


Polio is a disease caused by a tiny virus called Poliovirus. There are 3 types of wild poliovirus (WPV) strains. They differ from each other by their gene structure.

  • Type -1 poliovirus (WPV1)
  • Type -2 poliovirus (WPV2)
  • Type -3 poliovirus (WPV3)

Type -1 poliovirus is the most prevalent type in general. In fact this is the only wild poliovirus strain that continues to exist today. Rest 2 strains type – 2 ( WPV2) and type – 3 (WPV3) are eradicated after worldwide efforts. 

As per CDC (Centre For Disease Control ) 2 countries, Afghanistan and Pakistan still show endemicity for type – 1 poliovirus. Rest other countries have managed to keep type-1 poliovirus in check.

Why polio virus is dangerous?

Polio virus belongs to group of entero-virus. The word ‘entero’ relates to ‘intestine’. So no wonder why poliovirus has strong affinity for gut tissue. And it primarily infects human gut or intestine. Small children are easy targets in general. 

In 1% out of all cases, this virus can cause disease in central nervous system ( brain and spinal cord ) too.

Spinal cord infection is dreadful as it can lead to irreversible paralysis of limb( leg or arm ). Poliovirus in-fact has selective preference for “motor neurons” in spinal cord, that supply human muscles. Motor neuron damage is the basis for paralysis. 

When this happens, there occurs weakness in arm or leg or even muscles of face. It can be so extreme, that a person or a child can be debilitated for life. When it involves muscles of breathing, then it is lethal. Infected person can not breathe without an external support like a portable ventilator or iron lung ( used in earlier days ). 

On rare occasions this virus ends up in brain infection, which is known as “encephalitis”. It can also infect outer membrane ( meninges ) that covers brain. This is called “meningitis”. Both encephalitis and meningitis can evoke seizure or fits, unconsciousness, coma etc. 

Who are prone to develop polio infection and disease ?

Polio dominantly affects children. Standard age group for polio is between 6 months to 3 years of age. Till first 6 months of life, a baby is protected, owing to maternal antibodies. 


When it comes to gender, this virus impacts male more up against female in a ratio of 3:1. 

People who dwell in areas with poor sanitation, improper sewage disposal and overcrowding are more likely to suffer from polio. 

Polio is almost eradicated from western hemisphere of world since year 1994. Even so, few countries in eastern hemisphere are still endemic for polio. 

How polio virus multiplies inside human body ?

Before going into details of how polio is spread, let us first find out where and how it multiplies.

Once poliovirus enters into human body, it first multiplies in throat. It then goes down and settles in gut where it further amplifies in number. These are then excreted out in stool or faeces of infected person. 


Some viruses also invade into gut wall and enter blood stream. This circulation of virus in blood is called “viremia”. 

In some cases it then gain access into spinal cord or brain and causes severe symptoms like paralysis, encephalitis, meningitis etc. 

How polio is spread from person to person ?

Man is the only live reservoir of poliovirus. It doesn’t stay inside any other animal or bird. It resides in human body, especially gut and multiplies here. 

Poliovirus can also stay alive in water for even up to 4 months. Furthermore it can sustain on surfaces for as long as 2 months. This virus is pretty sensitive to heat, and perishes when exposed to the same. 

Following are the two crucial mode how polio is spread from one person to another.

1. Faeco-oral route :

Principal mode of transmission of poliovirus is through faeco-oral route. What does it mean by the way ? ‘Faeco’ word is derived from the word faeces or stool. And ‘oral’ stands for mouth. 

As the virus proliferates inside human gut, it sheds off in stool. So when in an area, sanitation is out of place with improper sewage disposal, this virus contaminates water, food etc. Wherefore sanitation plays a major role in containment of polio. 

Below we have specified few ways, by which polio is spread through faeco-oral route to a person. These are also known as 6 “F”.

  1. Fluid – When water and milk are contaminated 
  2. Food – If food is not properly cooked or handled or stored. 
  3. Fruit and vegetables – Taking unwashed fruits and vegetables 
  4. Fingers – Lousy personal hygiene
  5. Fomites – Such as utensils and other kitchen tools
  6. Flies – Which sit and then offload the germ on food 

A person goes on shedding virus in stool for several weeks following infection. 

2. Droplet :

This droplet transmission occurs in initial days of, course of disease. To begin with, virus inhabits throat and multiplies. During that period if that person coughs or sneezes then droplets are released into air. 

A person in close proximity inhales those infected droplets. This type of droplet transmission is infrequent in polio. It occurs only during first 1-2 weeks of infection. Use of mask prevents droplet transmission. 

History of polio

In earlier days, polio outbreaks were widespread across all countries. In the year 1952 around 21,000 paralytic polio cases were detected in U.S. itself. Worst hit age group were children below 5 years of age. Situation was

so alarming that people even feared to go to public places. 

Then there occurred discovery of IPV ( injectable polio vaccine ) in 1955 and OPV ( Oral Polio Vaccine ) in 1961. Can you imagine what were numbers of reported cases of polio in US in 1965? It is just 61 cases. This was achievable because of vaccine and sanitation. 

Though western hemisphere was slowly moving towards eliminating polio, but it persisted in many other parts of world. In year 1988  around 125 countries were still endemic to polio. Such that number of paralytic polio cases worldwide in 1988 were around 3,50,000 per year.

There came global initiative lead by WHO to eradicate polio. This was high-priority because even one case of paralytic polio in a child, was a threat to paralysis even in children of polio free countries. 

After world wide vaccination and other programs, number of polio cases got lowered significantly by 99%. As a result in the year 2021 only 6 cases were reported globally

What is current status of Polio ?

Western hemisphere was free from polio by year 1994

In the year 1999 major breakthrough happened when no cases of wild type poliovirus 2 ( WPV 2) was detected further. Following which, OPV that was formerly trivalent, was made bivalent. That means OPV which used to contain all 3 strains of polio namely WPV 1, WPV 2, and WPV 3 has only two strains now, WPV 1 and WPV 3. 

In year 2000 another striking change was brought in US where OPV was completely replaced by IPV which is going on till date. Also UK used IPV exclusively instead of OPV from year 2004. Likewise many developed counties have now completely switched to IPV instead of OPV. 

So the current eradication status of 3 wild types of poliovirus is chronologically as below.

Type of poliovirus 

Last case was detected in year

Received certificate of eradication in year

WPV 2 



WPV 3 



WPV 1 


Not yet eradicated*

*WPV1 is eradicated from most countries around globe, baring just 2 countries namely Pakistan and Afganistan. As international travel is common, polio still remains a threat to polio free countries as well. 

What are symptoms of polio ?

Symptoms of polio are varied. It ranges from very mild flu like illness to extreme debilitating paralysis. As per CDC ( Centre of Disease Control ) polio can present with either of 4 types mentioned below. 


In almost 70% cases of polio infection there is absolutely no symptom. These cases are the reason why it is so challenging to eradicate polio.

Around 24% cases of polio infection present with just fever, vomiting, body ache and flu like symptoms. Patient gets better on its own.

In 1-5% of cases, there is pain and stiffness in neck and back of spine. This is attributed to meningitis. 

<1% cases comes up with paralysis of limb due to spinal cord involvement. The affected limb ( leg or arm ) becomes weak and thin. 

Bulbar polio

Bulbar polio – Bulbar polio accounts for around 2% of all paralytic polio. In bulbar polio there is involvement of brain-stem. Brain stem is that structure which bridges brain and spinal cord.

Brain stem is the part from where cranial nerves arise and supply muscles of face, muscles of chewing/ swallowing, muscles of speech etc. Hence in bulbar polio patient incurs difficulty in swallowing and speaking. 

Breathing failure and cardiac failure are most serious and in fact potentially fatal outcome of severe Bulbar polio.

Bulbo-spinal polio is the combination of bulbar and spinal polio

How to prevent spread of polio and its infection ?

Remember polio has no cure, but is totally preventable. it can be done in following ways.


The main factor here is to maintain sanitation and hygiene. This calls out for environmental hygiene and personal hygiene also. 


Vaccine is the best mode of prevention for Polio. OPV ( Oral Polio Vaccine ) is given as 2 drops to child per mouth. On the other hand, IPV ( Inactivated Polio Vaccine ) is given as 0.5 ml injection into muscle. Both prevent paralysis in the wake of polio.

These vaccines are given as part of regular Immunization schedule of a child. Additionally mass polio immunisation is done on a particular day when all children of a country who are under 5 year are vaccinated. Door to door immunisation is another way to make sure no child is missed. 

As per data paralytic poliovirus cases have reduced significantly from year 1988 till date, by almost 99%. That’s huge.


3.Environmental surveillance

Surveillance of environment for poliovirus especially sewer water is done customarily. 

4.AFP surveillance

If a person is ill with symptoms similar to polio, then samples are collected from that patient to detect poliovirus. Each such case is evaluated and recorded. This is called AFP ( Acute Flaccid Paralysis ) Surveillance. 



Fighting paralytic polio is a marathon. We are almost there close to finishing line, just few steps away. It is high time we come together and eradicate polio once and for all, like we did for small pox. 


All the information provided here are for educational and awareness purpose only. Kindly do not use these as alternative to medical consultation.

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